A device of this type, which is provided as a change device for pictures, diapositives, advertising texts or the like, is known from DT-PS No. 873 908. This known change device is not suitable, however, for the reliable change of relatively thin sheets, which in addition have relatively large dimensional tolerances, such as, for example, normal photographs.
The difficulties involved in creating a photo-viewing change device of the type mentioned at the beginning, which functions reliably, is constructed as simply as possible and can be manufactured at a favourable cost, arise in particular from the fact that the photographs that are alternately to be exposed to view by such a photo-viewing change device are, in spite of their identical format, in many respects different, depending on which manufacturers have supplied the paper and chemicals for developing the photographs and depending on which printing company has produced the finished photographs therefrom.
The photographs supplied by the printing firms are not only of different sizes, the final sizes of 9.times.9 cm (more precisely 8.9.times.8.9 cm), 9.times.11.5 cm and 9.times.11.7 cm as well as 9.times.13 cm being the most customary, but within these nominal final sizes relatively large dimensional tolerances exist which may amount to 5 mm and which result from the fact that the photographs are made into the final size in a wide variety of methods and manners, such as, for example, by punching out, cutting up, cutting off from paper rolls etc. Furthermore, the thickness of the photographs can vary widely depending on the manufacturer. Photographs are, for example, generally between 0.25 and 0.3 mm thick, but may also be thicker or thinner than this.
Also, the photographs may have rectangular or rounded corners and the edges of the pictures may be differently formed, depending on the manner in which the picture is made into its final size, and may, where applicable, have ridges resulting from cutting.
Finally, the different photographs may be of different strengths and curved in different directions, since the so-called photographic paper does not, in fact, consist of paper, but comprises a plastics carrier which looks like paper on which a large number of different chemical layers is disposed which are stored or embedded in gelatine. Gelatine is a natural product and its properties therefore vary enormously. In particular, gelatine is relatively strongly hygroscopic whereas the plastics carrier is considerably less hygroscopic, so that depending on the air humidity, tensions of varying strength arise between the plastics carrier and the gelatine layers resulting in the very wide variety of curvature of photographs. This curvature is furthermore additionally influenced by the mentioned chemical layers, which are embedded in the gelatine and consist of at least one layer for protection against light, one filter layer, one fixing layer, one protective layer and three color layers and optionally further layers depending on which manufacturer has supplied the paper and which special chemical processes have been used for the development.